pants-plugins/uses_services: respect ST2_DATABASE__* vars

[cognifloyd]

This PR's commits were extracted from #6273 where I'm working on getting pants+pytest to run integration tests.

This lightly refactors pants-plugins/uses_services to make it easier to mention the ST2_* conf vars (introduced in #6277) in the instructions messages that the plugin prints when a service isn't running.

Then this PR updates the mongo rules to make use of the ST2_DATABASE__* conf vars if present. Thus, people can use ST2_DATABASE__* both in production and when testing to use whatever database they choose.

This only passes a subset of the possible ST2_DATABASE__* vars to tests. If anyone wants to add others, they need to review the tests to see if the tests override that setting or not. This is the config that tells pants to pass these vars to pytest:

st2/pants.toml

Lines 238 to 249 in 5c384d9

	[test]
	extra_env_vars = [
	# Use this so that the test system does not require the stanley user.
	# For example: export ST2TESTS_SYSTEM_USER=${USER}
	"ST2TESTS_SYSTEM_USER",
	# Use these to override MongoDB connection details
	# "ST2_DATABASE__HOST", # Tests override this with "127.0.0.1"
	"ST2_DATABASE__PORT",
	# "ST2_DATABASE__DB_NAME", # Tests override this with: "st2-test{ST2TESTS_PARALLEL_SLOT}"
	"ST2_DATABASE__CONNECTION_TIMEOUT",
	"ST2_DATABASE__USERNAME",
	"ST2_DATABASE__PASSWORD",

For background context on pants-plugins/uses_services see:

• Add pants-plugins/uses_services to check before running tests for required services (like mongo) #5864
• Update pants-plugins/uses_services to support checking for rabbitmq #5884
• Update pants-plugins/uses_services to support checking for redis #5893
• Pants: Misc pants-plugins/uses_services improvements #5898
• pants: Add system_user detection to pants-plugins/uses_services (+ mongo detection improvements) #6244